OBSERVATORY OF ULUGBEK

The construction of the observatory began in 1424 under the leadership of the grandson of Amir Tumir - Ulugbek during his reign of the Timurid Empire, and by 1428-1429 the building of the observatory was ready.

The observatory had the appearance of a three-story cylindrical building 30.4 m high and 46.40 m in diameter, and contained a grandiose goniometer oriented from north to south - a sextant (or quadrant) with a radius of 40.21 m, on which measurements were made of the height of celestial bodies above the horizon during the passage them through the celestial meridian. The device was discovered by excavations and is well preserved in the underground part. It is assumed that its arc was one sixth of a circle with a working part from 20° to 80°.

The arc of the instrument is limited by two barriers lined with marble. At each degree of the circle, divisions and numbers are carved on the marble. Each degree corresponds to an interval of 70 cm. There are brick stairs along the barriers.
Azimuthal observations could be made using a horizontal circle on the roof of the building. There were other instruments in the observatory that have not been preserved.
Here, by 1437, the Gurgan zij was compiled - a catalog of the starry sky, in which 1018 stars were described. The duration of the sidereal year was also determined there: 365 days, 6 hours, 10 minutes, 8 seconds (with an error of + 58 seconds).